The present invention relates to an electrophotographic photoconductor. More particularly, it relates to an electrophotographic photoconductor having an excellent durability in which a particular binder resin is used.
Electrophotography, because of its instantaneity and capability of forming high-quality images, has been widely used and applied in recent years not only in the field of duplication but also in the field of various types of printing. Regarding the photoconductor in the art of electrophotography, there have been widely used inorganic photoconductive materials such as selenium, arsenic-selenium alloy, cadminum sulfide, zinc oxide, etc., as main component material of the photoconductor, and more recently, there have been used organic photoconductive materials having advantages in pollution-free and good film-forming properties and easy production. As the organic photoconductors, there are known the so-called dispersion-type photoconductors in which photoconductive fine particles are dispersed in a binder resin, and the laminate-type photoconductors in which a charge-generation layer and a charge-transport layer are laminated.
In view of the facts that a photoconductor of high sensitivity can be obtained by combining a high-efficiency charge-generation material and a high-efficiency charge-transport material, that the laminate-type photoconductor has a wide scope of choice for component materials and is high in safety in use, and that these are also high in coating productivity and relatively low in cost, it is considered that probability is high for the laminate-type photoconductors to dominate the market, and studies are being made for the development and practical use of this type of photoconductor.
However, the currently available laminate-type photoconductors are inferior in durability to the inorganic type. The photoconductor are poor in physical property which is one of factors that determine durability, that is, they have the disadvantage that the photoconductor are susceptible to abrasion and surface scratches due to various loads applied in use, such as development with toner, friction with paper and abrasion by the cleaning members in which the load thereof varies depending on the method, so that the printing durability of the photoconductors is limited in practical use.
Generally, in the case of laminate-type photoconductor, these loads are applied to the charge-transport layer. The charge-transport layer is usually composed of a binder resin and a charge-transport material. Although the strength of the layer is substantially decided by the binder resin, the layer is not provided with a satisfactory mechanical strength due to the high doping amount of the charge-transport material.
As the binder resin for the charge-transport material, there have been used various types of thermoplastic and thermosetting resins such as polymethyl methacrylate, polystyrene, vinyl polymers such as polyvinyl chloride, copolymers thereof, polycarbonates, polyesters, polysulfones, phenoxy resins, epoxy resins, silicone resins, etc. Among the available binder resins, polycarbonates have comparatively excellent properties, and various types of polycarbonates have been developed and put to practical use. For example, Japanese Patent Application Laid-Open (Kokai) No. 50-98332 (1975) discloses bisphenol P-type polycarbonates, and Japanese Patent Application Laid-Open (Kokai) No. 59-71057 (1984) discloses bisphenol Z-type polycarbonates. Also, Japanese Patent Application Laid-Open (Kokai) No. 60-184251 (1985) proposes use of copolymer-type polycarbonates of bisphenol P and bisphenol A as a binder resin.
Further, Japanese Patent Application Publication (Kokoku) No. 48-38430 (1973) discloses the heterogeneous photoconductive compositions (disperse layer) comprising an organic pigment and a polymer, and teaches that these heterogeneous photoconductive compositions can be produced with good reproducibility by using various types of polycarbonates as the polymer. But there is nothing about the mechanical properties such as wear resistance of these compositions.
However, when a laminate-type photoconductor made by using such heterogeneous photoconductive composition is used in a high-speed electrophotographic process, in most cases it is unsatisfactory in abrasion and scratch resistances. Thus, the development of a binder resin having higher qualities has been desired.
As a result of the present inventors' extensive studies on binder resin used for the photosensitive layer, it has been found that in an electrophotograpahic photoconductor having at least a charge-generation layer and a charge-transport layer on a conductive substrate, by using as a binder resin a particular polycarbonate composed of at least one structural unit represented by the formula I described later and at least one structural unit represented by the formula II described later, the obtained electrophotographic photoconductor shows well satisfactory mechanical and electrical properties with excellent stability.